Method of improving the output uniformity of a display device

ABSTRACT

This invention relates to a method of improving the output uniformity of a display device ( 1 ), such as a self light emitting display device, comprising the following steps; detecting a first emitted brightness of at least one pixel ( 5 ) of said display device ( 1 ); by means of the detected first brightness, determining the non-uniformity of an output of a driver circuit ( 3 ) being connected with said at least one pixel ( 5 ); and based on said first detected brightness, generating a calibration factor for the at least one pixel ( 5 ), to be used to modify the output of the driver circuit ( 3 ), in order to improve the uniformity.

This invention relates to a method of improving the output uniformity ofa display device, preferably a self light emitting display device, andmost preferably an organic light emitting diode based display device.The invention also relates to a system implementing the method and to adisplay for use with said system.

Recently, the interest for self light emitting display devices has beenincreasing. For example, self light emitting display devices, whichutilises self light emitting materials, such as polymer or organic lightemitting materials, have been found to be a potential substitute forother display types, such as liquid crystal displays or cathode raytubes.

Basically, a self light emitting display device, such as a polymer lightemitting diode display or an organic light emitting diode displaycomprises a plurality of pixels, each containing self light emittingmaterial, and a driving structure, for applying a driving current to theself light emitting material. Commonly, the device comprises a matrix ofpixels arranged on a substrate, such as a glass or polymer substrate.The matrix structures may essentially be sub-divided into two maingroups, passive and active matrix structures. In a passive matrixpolymer or organic light emitting display, a layer of light emittingmaterial is arranged between a row electrode layer and a columnelectrode layer being intersecting (see FIG. 1) and thus forming pixels.Typically, the display emission is controlled by means of data drivers,each controlling the current through a column. In an active matrixpolymer or organic light emitting display (see FIG. 2), each pixel of apixel matrix is controlled by means of pixel driving circuit. Moreover,each column is controlled by means of a data driving circuit.

However, a problem with active matrix polymer light emitting diodedisplays, using p-Si thin film transistors is that variations of thecharacteristics of such transistors result in a random pixel-to-pixelvariation of the brightness of the display, resulting in non-uniformityof the display output. This variation is particularly strong for themost simple trans-conductance circuits, having two thin film transistorsper pixel, in which a drive thin film transistor is used to convert anaddressing voltage to a driving current. Examples of such circuits areshown in FIG. 3 and FIG. 4. Therefore, this type of circuit isunsuitable for high-performance displays. However, for other reasons,the most simple trans-conductance circuits are preferred, since theyprovide a high pixel aperture, may be addressed very quickly, even atlow brightness levels and are also the most simple to address, sincethey essentially may use established drivers, similar to the ones usedin liquid crystal displays. To overcome the above problems wouldtherefore be advantageous.

An additional problem is that current generating data drivers forAMP(O)LED devices are not readily available at this point. One reasonfor this is again the requirement of high uniformity; if any one of thedriver outputs has a different current value, this will be instantlyrecognisable as a bright or dark line running through the display. Forthis reason, uniformity of the driver is even more essential thanuniformity of the pixels themselves, where the randomness of theuniformity variations renders the visibility somewhat lower. One way tosolve this problem that has been proposed is to use drivers of a selfcompensating current mirror type. However, this solution is complex andrequires much space, and moreover, such drivers are slow to address andare less accurate at lower current levels, and they also require higherdriving voltages, hence consumes more power. As an alternative, simplerand less complicated drivers, such as a 2 TFT transconductance driver,may be used, but as indicated above, they have an unacceptablenon-uniformity.

Moreover, also in the case of a passively driven self light emittingdisplay device, non-uniformity of the data driver output is a problem,in much the same way as described above.

Hence, a general method for improving or overcoming non-uniformityissues in display devices in general and in display devices inparticular is desired, and an object of this invention is consequentlyto achieve such a method, overcoming the disadvantages with the priorart, as indicated above.

This and other objects is at least in part achieved by a methodaccording to claim 1. According to this method, the output uniformity ofa display device is improved by; detecting a first emitted brightness ofat least one pixel of display device; by means of the detected firstbrightness, determining the non-uniformity of an output of a drivercircuit being connected with said pixel; and, based on said firstdetected brightness, generating a calibration factor for the at leastone pixel, to be used to modify the output of the driver circuit, inorder to improve the uniformity. In this manner, non-uniformity in thedisplay emission, resulting from variations in a single devicecharacteristics which scales linearly with the light output, may becompensated for. By measuring the pixel output at differentbrightnesses, it is possible to distinguish uniformity variations fromdifferent sources. Preferably, the method may be used for self lightemitting display devices, and more preferably for organic light emittingdiode based display devices.

More generally, several factors will contribute to the non-uniformity ofthe display light output, including variations in the performance oftransistors and other electrical components in the driving circuit, andalso variations in the efficiencies of the light emitting devicesthemselves. Therefore, according to a preferred embodiment of theinvention, the method further comprises the step of, after detectingsaid first emitted brightness, adjusting an average display brightness,and thereafter detecting a second emitted brightness of said at leastone pixel, and based on said first and second detected brightnesses,generating a calibration factor for the at least one pixel, to be usedto modify the output of the driver circuit, in order to improveuniformity. In this manner, non-uniformity in the display emissionresulting from variations from more than one device characteristics maybe compensated for. By measuring the pixel output at differentbrightnesses, it is possible to distinguish uniformity variations fromeven more different sources.

The step of detecting the emitted brightness of at least one pixel issuitably performed by means of an external imaging system. An example ofsuch an external system is a CCD camera based system. Hence, afabricated display is positioned under such an external imaging system,where after the display is calibrated by using the inventive method inorder to improve the output uniformity of the display. Preferably, saiddriver circuit is one of a pixel driver circuit or a data drivercircuit, depending on the display construction.

According to a first preferred embodiment of the invention, said displaydevice is a active matrix polymer or organic light emitting diodedisplay device. In this case, the brightness may either be detectedindividually for each pixel, or simultaneously for an entire row orcolumn of pixels, as will be further described below. However, accordingto one embodiment of the invention, the step of detecting the emittedbrightness of at least one pixel comprises the step of individuallydetecting the emitted brightness for each of a plurality of pixels,being a straight-forward application of the invention on pixel level.Alternatively, the step of detecting the emitted brightness of at leastone pixel comprises the step of jointly measuring the emitted brightnessof a group of pixels, such as a column or a row of pixels, beingcommonly controlled by a common driving device. This embodiment has anumber of advantages over the pixel level embodiment described above.First, column level compensation removes a more visible artefact, as isindicated above. Moreover, as stated above, less memory is required(about 100-1000 times less, representing the number of rows on a typicaldisplay and also smaller look-up tables is required, in embodimentsusing such tables. Further, this embodiment enables the use of moresimple current driver circuits, since the uniformity demands on suchcircuits may be lowered. Thereby, faster components, having a lowerpower consumption and/or a smaller size may be used. Furthermore, thisembodiment may be used for all brightness levels, as generating lowoutput current values no longer requires low programming currents, whichmakes programming slow, but now may be implemented by programming onlyvoltages, which is faster. In addition, this embodiment is also fasterto implement, since less data is to be loaded into look-up tables and soon.

In order to further improve the output uniformity of an active matrixdisplay device, the method may further comprise the step of aligning, inone of a column or a row of pixels, all transistors of all pixels in adirection, being the direction of a laser beam during a laserrecrystallisation step during the fabrication of said transistors.

According to another preferred embodiment of this invention, saiddisplay device is a passive matrix polymer or organic light emittingdiode display device. In the same way as above, for the active matrixembodiment, the step of detecting the emitted brightness of at least onepixel suitably comprises the step of jointly measuring the emittedbrightness of a group of pixels, such as a column or a row of pixels,being commonly controlled by a common driving device.

Also, said calibration factors are preferably memorised in the drivercircuit for the pixel, column or row, or in the display controller, byone of the methods; storing the calibration factors in a memory device,burning fuses on one of a transistor substrate or an additional driverintegrated circuit, or laser trimming of one of a transistor substrateor an additional driver integrated circuit.

The above and other objects of the invention are also at least partlyachieved by a system for calibrating a display device, for improving theoutput uniformity of the same, comprising a unit for holding a displaydevice to be calibrated, an imaging system, being positioned so as to,when in use, detecting emitted brightness from the entire display devicesurface of the display device, and a feedback system, for transmittinginformation regarding the emitted brightness back to the display device,the system being arranged to perform the inventive method describedabove. Preferably, the display device to used with the system is a selflight emitting display device, preferably an organic light emittingdiode based display device.

Also, the above and other objects of the invention are also at leastpartly achieved for use with a system as defined above. According to apreferred embodiment, the display device further comprises a pluralityof light emitting pixels being arranged in a row and column structure,wherein either each column or each row of pixels being connected with adata driver circuit, wherein each column or each row comprises anadditional non-light emitting pixel, incorporating a current measurementdevice, for monitoring a relative change over time of an output signalfrom said data driver.

The invention will hereinafter be described in closer detail, by meansof preferred embodiments thereof, with reference to the accompanyingdrawings.

FIG. 1 is a schematic drawing of the basic configuration of a passivematrix polymer or organic light emitting diode display, essentiallycomprising a matrix of intersecting row and column electrodes, whereby alayer of polymer or organic light emitting material is sandwichedbetween a layer of row electrodes and a layer of column electrodes.

FIG. 2 is a schematic drawing of the basic configuration of an activematrix polymer or organic light emitting diode display.

FIG. 3 is a schematic drawing of a first current source circuit, thatmay be used in the display disclosed in FIG. 2.

FIG. 4 is a schematic drawing of a second current source circuit, thatmay be used in the display disclosed in FIG. 2.

FIG. 5 schematically discloses a basic explanatory system embodying thepresent invention.

FIG. 1 schematically discloses a passive matrix polymer or organic lightemitting display device for which the present invention may be used. Inthe disclosed passive matrix polymer or organic light emitting display,a layer of light emitting material is arranged between a row electrodelayer 8 and a column electrode layer 7 being intersecting (see FIG. 1)and thus forming pixels 5.

FIG. 2 schematically discloses an equivalent circuit diagram of a partof an active matrix polymer or organic light emitting display device 1,for which the present invention may be used. This display devicecomprises a matrix of (P) LEDs or (O)LEDs with m rows (1, 2, . . . , m)and n columns (1, 2, . . . , n). Where rows and columns are mentioned,it shall be noted that they may be interchanged, if desired. The devicefurther comprises a row selection circuit 16, connected with said rowsand a data register 15 connected with said columns. Externally presentedinformation 17, for example, a video signal, is processed in aprocessing unit 18 which, dependent on the information to be displayed,charges separate parts 15-1, . . . 15-n of the data register 15 viasupply lines 19. The selection of row takes place by means of the rowselection circuit 16, via the row lines 8, in this case gate electrodesof transistors 22, such as TFT transistors, by providing them with therequired selection voltage. Writing data takes place in that, duringselection, data signals are provided from the data register 15, in thiscase in the form of voltage signals. During addressing, a capacitor 24is charged to the level of the data voltage via the transistors. Thiscapacitor determines the adjustment of the transistor 21 and hence theactual current through the LED 20 during a driving period and theluminance of the pixel. Synchronisation between the selection of rows 8and the presentation of voltages to the columns 7 takes place by meansof the processing unit 18, via drive lines 14.

The basic idea behind the invention will hereinafter be described,followed by a number of preferred embodiments thereof. A basicexplanatory system embodying the present invention is disclosed in FIG.5, for illustrative purposes only. Here, a fabricated self lightemitting display 1 is arranged to be adjusted in order to improve theoutput uniformity of the light-emitting elements of the display. Thedisplay device may for example be a passive matrix polymer or organiclight emitting display, as described above and as schematically shown inFIG. 1, or an active matrix polymer or organic light emitting display(AMP(O)LED) as described above and as schematically shown in FIG. 2.

The fabricated display device 1 is positioned under an external imagingsystem 2. This system may for example be a CCD camera-based system, ableto detect light emitted from the display device 1. Thereafter thedisplay device 1 is addressed in order to emit light. The addressing maybe done one pixel at a time, one column at a time or one row at thetime, as will be further described below. The addressing is made bymeans of a driver circuit 3. The driver circuit may be a pixel drivercircuit, as disclosed in FIG. 5, (active matrix configuration only), inwhich one driver circuit is arranged for each pixel of the displaydevice 3. Examples of such circuits are disclosed in FIG. 3 and FIG. 4.Alternatively, the driver circuit is a data driver circuit, (applicablefor both passive and active matrix configuration), in which one drivercircuit 3 is arranged for each pixel column of the display, to controlthe column of pixels. In any case, all driver circuits of the displayare connected with a central processing unit/a controller unit of thedisplay (not shown) being used to provide information to each drivercircuit about what driver circuits is to be addressed at a certain time.

However, in the example disclosed in FIG. 5, one pixel of the display 1is addressed at a time, whereby the pixel emits light, having abrightness depending on an output signal 4 to the driving circuit 3. Theemitted brightness from the pixel is thereafter detected by the externalimaging system 2, where after the detected brightness is fed back to thedriver circuit 3 (or, alternatively, to a separate processing unit,connected to the driver circuit), via a feedback unit 6. In the drivercircuit 3 (or the separate processing unit), the detected brightness iscompared with a desired brightness for the output signal 4 in question,and the display non-uniformity of that specific output signal 4 may beestablished by signal processing. Thereafter, if required, the outputsignal 4, and hence the emitted brightness is adjusted, and the abovedetection is repeated, one or more times. From the values obtained bythose measurements, the non-uniformity of essentially each possiblevalue of the output signal may be established by means of interpolation,and from these values, a calibration factor, adjusting each outputsignal for achieving a desired pixel output is calculated. Thiscalibration factor is thereafter stored in the driver circuit or in anassociated circuit. This may be made by storing the calibration factorin a memory or by adjusting the hardware, for example by burning fusesor use laser trimming on the driver circuit or an associated circuit.

In order to achieve calibration factors for all pixels of the display,the above process is repeated for all pixels of the display, and in thecase of a full-colour display, for each colour of the display.Alternatively, an entire display, i.e. all pixels of the display, may beaddressed simultaneously with a calibration image, and in this case, theoutput of all pixels are measured simultaneously by the imaging system2.

The above method may equally well be used on a column/row level. Howeverin this case, an entire column or row is addressed at once, and theintegral brightness of all pixels along the column/row is detected. Thecalibration factors are in this case implemented in a data drivercircuit, instead of in the pixel driver circuit. Also in this case, anentire display, i.e. all pixels of the display, may be addressedsimultaneously with a calibration image, and in this case, the output ofall columns/rows are measured simultaneously by the imaging system 2.

EMBODIMENT 1

According to a first embodiment of this invention, the inventive methodis implemented at pixel level in a AMP(O)LED display. A fabricatedAMP(O)LED display is placed under an imaging system, such as a CCDcamera based system. The display is turned on so that the pixel that isto be studied emits light (the process is repeated for all pixels of thedisplay that is to be studied. Alternatively, all pixels could beaddressed at once, as described above). The brightness of the pixel isdetermined, and the determined brightness is thereafter compared with adesired brightness for the given driving input to the pixel. By thiscomparison, a measure for the non-uniformity of the pixel circuit outputis determined. Examples of situations where a correction based on thisnon-uniformity measure is sufficient are for pixel driving circuitswhere only variations in the mobility of individual transistors definethe non-uniformity, or where the variation in the efficiency of thelight emitting device itself is responsible for the non-uniformity ofthe display brightness. The above process is repeated for all pixels,and also for all colours in a full colour display.

Subsequently, the measure of the non-uniformity of the pixel output isused to calculate a calibration factor, which is stored in a full framememory in the display device, the memory being connected to the drivecircuit of the pixel. If desired, a look-up table may be generated fromthe derived factors in order to derive calibration factors for differentbrightness levels. The calibration factors stored in the memory, or thefactors derived from the look-up table or an analytical function, as thecase may be, is hereafter used to modify the input to the pixel driverin order to maintain uniformity in all pixels at all brightness levels.Signal processing approaches for such modifications are known in theprior art.

EMBODIMENT 2

According to a second embodiment of this invention, the inventive methodis implemented at pixel level in a AMP(O)LED display. A fabricatedAMP(O)LED display is placed under an imaging system, such as a CCDcamera based system. The display is turned on so that the pixel that isto be studied emits light (the process is repeated for all pixels of thedisplay that is to be studied. Alternatively, all pixels could beaddressed at once, as described above.) The brightness of the pixel isdetermined, and the determined brightness is thereafter compared with adesired brightness for the given driving input to the pixel. By thiscomparison, a measure for the non-uniformity of the pixel circuit outputis determined. The above process is repeated for all pixels, and alsofor all colours in a full colour display.

Thereafter, the average display brightness is adjusted, where after theabove process is repeated, and hence the pixel brightness is remeasured.The process may be repeated several times, if desired, each timemeasuring at a different brightness level.

When measuring the pixel output at different brightnesses it is possibleto distinguish uniformity variations from different sources. Forexample, for a transconductance pixel circuit, both TFT mobility (μ) andTFT threshold voltage (V_(th)) variations contribute to the brightnessof the pixel in different manners following the following relationship:I∝μ·(V−V _(th))²   (1)

In addition, non-uniformity resulting from variations in the technology,or degradation of emitting devices may be eliminated by extension ofthis method to further brightnesses.

Subsequently, the measure of the non-uniformity of the pixel output isused to calculate a calibration factor, which is stored in a full framememory in the display device, the memory being connected to the drivecircuit of the pixel. Alternatively, the values of μ, V_(th), etc. maybe stored in the memory. If desired, a look-up table may be generatedfrom the derived factors in order to derive calibration factors fordifferent brightness levels. The calibration factors stored in thememory, or the factors derived from the stored parameters, a look-uptable or an analytical function, as the case may be, is hereafter usedto modify the input to the pixel driver in order to maintain uniformityin all pixels at all brightness levels. Signal processing approaches forsuch modifications are known in the prior art.

EMBODIMENT 3

This embodiment is similar to the one described under embodiments 1 and2, but in embodiment 3 the calibration factors are not stored in anadditional memory. Instead the calibration factors are introduced to thepixel driver by means of burning fuses or laser trimming of components.This may be done on the p-Si substrate, but may alternatively be made onan additional driver circuit, or circuits, being connected to the pixeldriver. The advantage of this embodiment is that it may be implementedat a comparatively low cost.

EMBODIMENT 4

According to a fourth embodiment of this invention, the inventive methodis implemented at data driver level in a AMP(O)LED display.

A fabricated AMP(O)LED display is placed under an imaging system, suchas a CCD camera based system. The display is turned on so that the pixelcolumn that is to be studied emits light (the process is repeated forall columns of the display that is to be studied. Alternatively, allcolumns may be addressed at once, as described above.) The brightness ofthe entire pixel column is determined, and the determined brightness isthereafter compared with a desired brightness for the given drivinginput to the column. By this comparison, a measure for thenon-uniformity of the data driver circuit output, resulting from avariation in a single device characteristic which scales linearly withthe light output, is determined. Examples of situations where such acorrection will be sufficient are for data driving circuits where onlyvariations in the mobility of individual transistors define thenon-uniformity. The above process is repeated for all columns, and alsofor all colours in a full colour display. By studying an entire columnat once, the effect of random brightness variation of individual pixelsis minimised.

Subsequently, the measure of the non-uniformity of the pixel columnoutput is used to calculate a calibration factor, which is stored in acomparatively small memory (since only one calibration factor is neededper column, instead as per pixel as in embodiment 1) in the displaydevice, the memory being connected to the drive circuit of the pixelcolumn. Alternatively, the values of μ, V_(th), etc. may be stored inthe memory. If desired, a comparatively small look-up table, as comparedto embodiment 1, may be generated from the derived factors in order toderive calibration factors for different brightness levels. Thecalibration factors stored in the memory, or the factors derived fromthe stored parameters, a look-up table or an analytical function, as thecase may be, are hereafter used to modify the input to the data driverin order to maintain uniformity in all columns at all brightness levels.Signal processing approaches for such modifications are known in theprior art.

As compared to the pixel level compensation, described under embodiment1, the column level compensation described under embodiment 4 has aplurality of advantages. First, column level compensation removes a morevisible artefact, as is indicated above. Moreover, as stated above, lessmemory is required (about 100-1000 times less) and also smaller look-uptables are required, in embodiments using such tables. Further, thisembodiment enables the use of more simple current driver circuits, sincethe uniformity demands on such circuits may be lowered. Thereby, fastercomponents, having a lower power consumption and/or a smaller size maybe used. Furthermore, as explained above, this embodiment may be usedfor all brightness levels, and it is also faster to implement, sinceless data is to be loaded into look-up tables and so on.

EMBODIMENT 5

According to a fifth embodiment of this invention, the inventive methodis implemented at data driver level in a AMP(O)LED display.

A fabricated AMP(O)LED display is placed under an imaging system, suchas a CCD camera based system. The display is turned on so that the pixelcolumn that is to be studied emits light (the process is repeated forall columns of the display that is to be studied. Alternatively, allcolumns may be studied at once, as explained above. The brightness ofthe entire pixel column is determined, and the determined brightness isthereafter compared with a desired brightness for the given drivinginput to the column. By this comparison, a measure for thenon-uniformity of the pixel circuit output is determined. The aboveprocess is repeated for all pixels, and also for all colours in a fullcolour display. By studying an entire column at once, the effect ofrandom brightness variation of individual pixels is minimised.

Thereafter, the average display brightness is adjusted, where after theabove process is repeated, and hence the pixel column brightness isremeasured. The process may be repeated several times, if desired, eachtime measuring at a different brightness level.

Measuring the pixel column output at different brightnesses enablesdistinction of uniformity variations from different sources. Forexample, for a transconductance column driver, both TFT mobility (μ) andTFT threshold voltage (V_(th)) variations contribute to the brightnessof the pixel in different manners following the same relationship asdefined by equation (1).

Subsequently, the measure of the non-uniformity of the pixel columnoutput is used to calculate a calibration factor, which is stored in acomparatively small memory (as compared to embodiment 1) in the displaydevice, the memory being connected to the drive circuit of the pixelcolumn. Alternatively, the values of μ, V_(th), etc. may be stored inthe memory. If desired, a small look-up table may be generated from thederived factors in order to derive calibration factors for differentbrightness levels. The calibration factors stored in the memory, or thefactors derived from the stored parameters, a look-up table or ananalytical function, as the case may be, is hereafter used to modify theinput to the data driver in order to maintain uniformity in all columnsat all brightness levels. Signal processing approaches for suchmodifications are known in the prior art.

As compared to the pixel level compensation, described under embodiment1, the column level compensation described under embodiment 3 has aplurality of advantages. First, column level compensation removes a morevisible artefact, as is indicated above. Moreover, as stated above, lessmemory is required (about 100-1000 times less) and also smaller look-uptables is required, in embodiments using such tables. Further, thisembodiment enables the use of more simple current driver circuits, sincethe uniformity demands on such circuits may be lowered. Thereby, fastercomponents, having a lower power consumption and/or a smaller size maybe used. Furthermore, this embodiment may be used for all brightnesslevels, as explained above, and it is also faster to implement, sinceless data is to be loaded into look-up tables and so on.

EMBODIMENT 6

According to a sixth embodiment of this invention, the inventive methodis implemented in a further improved way at data driver level in aAMP(O)LED display.

While embodiments 4 and 5 described above provides a lower costimplementation, it does not removed pixel-to-pixel variations caused byvariations in the TFT performance. Examples of driving circuitscomprising TFTs are disclosed in FIG. 3 and FIG. 4. When manufacturing aTFT, details of the laser crystallisation step during the p-Sifabrication process results in a difference in performance of thecomponent, either along a laser scan direction or in the direction of alaser beam. In general, uniformity is higher along the laser beam andworse in its scan direction. Hence, according to the fourth embodimentof this invention, all drive TFTs for all pixels along a column of thedisplay is aligned in the direction of the laser beam. Thereby, theuniformity of the TFTs within the column will be as high as possible,whilst the variation between different columns will be large. The latteris however less important, as column-to-column variations may becorrected using the approach described under embodiment 3. In this way,a display having an improved pixel-to-pixel uniformity may be achieved,without increasing the cost as compared to the method described underembodiment 3.

EMBODIMENT 7

According to a seventh embodiment of this invention, the inventivemethod is implemented in an additional further improved way at datadriver level in a AMP(O)LED display.

In an alternative to embodiment 3 and in the same spirit as inembodiment 4, all drive TFTs of a row in a display device may be alignedin the direction of the laser beam during manufacture of the TFTs. Inthis case, the uniformity of the TFTs within a row will be as high aspossible, whilst the row-to-row variation will be large. In order tosolve this problem, it is in addition necessary to determine abrightness calibration factor for each row of the display. This may bedone in the corresponding way as defined under embodiment 3, but insteadinvestigating the integral brightness for each row. Thereafter, both thecolumn calibration factor, as obtained in accordance with embodiment 3,and the above-described row calibration factor are stored in thecorresponding way as in previous embodiments. In this case, column datawill be processed using the stored information of both the average rowand column calibration factors, based on the stored row and columncalibration factors. By this embodiment, a display with an improvedpixel-to-pixel uniformity may be achieved, having only a slight increaseof cost as compared to the approach suggested under embodiment 3.

EMBODIMENT 8

According to a eight embodiment of this invention, the inventive methodis implemented in yet a further improved way at data driver level in aAMP(O)LED display.

In the embodiments 3-5 described above, column (and row) calibrationfactors are stored in an additional small memory. However, according tothis embodiment, calibration may also be made by burning fuses or lasertrimming of components, in the same way as is described under embodiment2 for the pixel level implementation. This may be done on the p-Sisubstrate, but may alternatively be made on an additional drivercircuit, or circuits, being connected to the data driver. The advantageof this embodiment is that it may be implemented at a comparatively lowcost.

EMBODIMENT 9

All of the above embodiments address the problem of display uniformityat the start of the display lifetime, i.e. during manufacture or shortlythereafter. However, degradation of the p-Si TFTs during usage mayintroduce non-uniformities as the display is used. In order to avoidthis problem, a current measurement device may be added to each datadriver. Preferably, this may be achieved by adding a dummy pixel to eachcolumn, incorporating the current measurement device. The function ofthis current measurement device is to monitor any changes in the outputof the column during the lifetime of the display. It shall be noted thatit is only necessary to monitor a relative change of the output, i.e.the difference between the current output and the initially measuredoutput, as defined by the brightness measurements performed at the startof the display lifetime, in accordance with any one of theabove-described embodiments. The monitoring of the relative changeshould be performed occasionally, rather than constantly, in order toavoid distortion of the display operation and avoid causing degradationwithin the TFTs of the measuring circuit itself. Any monitored change inthe output triggers an update of the calibration factor for theappropriate data driver, for example by calculating and storing the newcalibration value in the appropriate memory spot.

EMBODIMENT 10

While the above embodiments are primarily focused on applying thepresent invention on a AMP(O)LED display, this embodiment describes theinventive method as implemented at data driver level for a passivepolymer or organic light emitting diode display (P(O)LED).

According to this embodiment, a fabricated passive P(O)LED display,including final driver integrated circuits is placed under an imagingsystem, such as a CCD camera based system. The display is turned on sothat the pixel column that is to be studied emits light (the process isrepeated for all columns of the display that is to be studied.Alternatively, all columns may be studied at once, as described above.)The integral brightness along the complete column is determined, and thedetermined brightness is thereafter compared with a desired brightnessfor the given driving input to the column. By this comparison, a measurefor the non-uniformity of the driver IC output is determined. The aboveprocess is repeated for all columns, and also for all colours in a fullcolour display. By studying an entire column at once, the effect ofrandom brightness variation of individual pixels is minimised.

Thereafter, the average display brightness is adjusted, where after theabove process is repeated, and hence the column brightness isremeasured. The process may be repeated several times, if desired, eachtime measuring at a different brightness level.

Subsequently, the measure of the non-uniformity of the column output isused to calculate a calibration factor, which is stored in a smallmemory in the display device, the memory being connected to the drivecircuit of the pixel column. Alternatively, the values of μ, V_(th),etc. may be stored in the memory. If desired, a small look-up table maybe generated from the derived factors in order to derive calibrationfactors for different brightness levels. Alternatively, the calibrationfactors may be “stored” in the device by burning fuses or use lasertrimming on the driver IC, in the corresponding way as described in theembodiments 2 and 6.

The calibration factors stored in the memory, or the factors derivedfrom the stored parameters, a look-up table or an analytical function,as the case may be, is hereafter used to modify the input to the datadriver in order to maintain uniformity in all columns at all brightnesslevels. Signal processing approaches for such modifications are known inthe prior art.

While this invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art. Inparticular, whilst several embodiments have been described in terms ofpolymer or organic LED based self light emitting displays, the inventionis equally applicable to other types of self light emitting displaydevices, such as field emission displays, plasma displays etc. and alsoto non-light emitting displays, for example light valve type displayssuch as liquid crystal displays. Accordingly, the preferred embodimentsof the invention, as set fourth herein, are intended to be illustrative,not limiting. Various changes may be made without departing from thespirit and scope of the invention as defined in the following claims.

1. A method of improving the output uniformity of a display device (1),comprising the following steps: detecting a first emitted brightness ofat least one pixel (5) of display device (1); by means of the detectedfirst brightness, determining the non-uniformity of an output of adriver circuit (3) being connected with said at least one pixel (5);based on said first detected brightness, generating a calibration factorfor the at least one pixel (5), to be used to modify the output of thedriver circuit (3), in order to improve the uniformity.
 2. A methodaccording to claim 1, wherein said display device is a self lightemitting display device.
 3. A method according to claim 1 or 2, whereinsaid display device is an organic light emitting diode based displaydevice.
 4. A method according to any one of the claims 1-3, furthercomprising the steps of: after detecting said first emitted brightness,adjusting an average display brightness, and thereafter detecting asecond emitted brightness of said at least one pixel (5), and based onsaid first and second detected brightnesses, generating a calibrationfactor for the at least one pixel (5), to be used to modify the outputof the driver circuit (3), in order to improve uniformity.
 5. A methodaccording to any one of the claims 1-4, wherein the step of detectingthe emitted brightness of at least one pixel (5) is performed by meansof an external imaging system (2).
 6. A method according to any one ofthe claims 1-5, wherein said driver circuit (3) is one of a pixel drivercircuit or a data driver circuit.
 7. A method according to any one ofthe claims 1-6, wherein said display device (1) is a active matrixpolymer or organic light emitting diode display device.
 8. A methodaccording to claim 7, wherein the step of detecting the emittedbrightness of at least one pixel (5) comprises the step of individuallydetecting the emitted brightness for each of a plurality of pixels.
 9. Amethod according to claim 7 or 8, further comprising the step ofaligning, in one of a column or a row of pixels, all transistors of allpixels in a direction, being the direction of a laser beam during alaser recrystallisation step during the fabrication of said transistors.10. A method according to any one of the claims 1-6, wherein saiddisplay device (1) is a passive matrix polymer or organic light emittingdiode display device.
 11. A method according to any one of the claims1-7 and 9-10, wherein the step of detecting the emitted brightness of atleast one pixel (5) comprises the step of jointly measuring the emittedbrightness of a group of pixels, such as a column or a row of pixels,being commonly controlled by a common driving device.
 12. A methodaccording to any one of the preceding claims, wherein said calibrationfactors are memorised in the driver circuit (3) by one of the methods;storing the calibration factors in a memory device, burning fuses on oneof a transistor substrate or an additional driver integrated circuit, orlaser trimming of one of a transistor substrate or an additional driverintegrated circuit.
 13. A system for calibrating a display device (1),for improving the output uniformity of the same, comprising a unit forholding a display device (1) to be calibrated, an imaging system (2),being positioned so as to, when in use, detect emitted brightness fromthe entire display device surface of the display device (1), and afeedback system (6), for transmitting information regarding the emittedbrightness back to the display device (1), the system being arranged toperform the method according to any one of the claims 1-12.
 14. A systemaccording to claim 13, wherein said display device (1) is a self lightemitting display device, preferably an organic light emitting diodebased display device.
 15. A self light emitting display device (1) foruse with a system as defined in claim
 13. 16. A self light emittingdisplay device (1) as defined in claim 15, wherein the display devicecomprises a plurality of light emitting pixels being arranged in a rowand column structure, wherein either each column or each row of pixelsbeing connected with a data driver circuit, wherein each column or rowcomprises an additional non-light emitting pixel, incorporating acurrent measurement device, for monitoring a relative change over timeof an output signal from said data driver.